The present invention relates generally to methods for manufacturing micro-machined electromechanical sensor (MEMS) devices, and in particular to methods for compensating cumulative variations during processing.
The manufacture of MEMS devices, such as accelerometers or rate sensors, is generally well-known and includes many individual processes. Each of the individual processes is a source of variation. The variations of the individual processes accumulate in significant performance differences between different devices manufactured using the same processes in the same way.
One means of coping with this cumulative variation is to control each of the processes in a serial manner to the degree necessary to maintain the cumulative variation within predetermined design parameters. Such process control can, however, be extremely time consuming and costly.
Another alternative coping mechanism is to compensate the behavior of the device after completion of all of the processing steps.
The present invention is a means of delicately adjusting the orientation of features in completed MEMS devices, which may or may not already have silicon cover plates applied to protect mechanism integrity. The use of such method allows the trimming of modulation, bias and other dynamic behaviors of the MEMS devices after initial formation and installation within the device packaging.
The method of trimming in the present invention exploits the ability of film stresses, created by growing a film of different thermal expansion coefficient on the MEMS silicon substrate, to change the shape of the MEMS device. After the film is grown on the substrate, laser radiation is used to selectively remove a portion of the film, such that the remaining material will upset the balance of stresses on the member and cause it to take on a different shape. A laser radiation frequency is selected such that the radiation will transmit through the substrate but will be absorbed by the film, causing removal of the film by ablation. The extent to which the trimming effect occurs depends on the amount of material removal desired, the location from which the material is removed, and the sensitivity of the MEMS design to removal of the material.
According to one aspect of the invention, a compensation method for MEMS devices is provided, the method including modeling predetermined performance parameters of a MEMS device; manufacturing the MEMS device in a silicon substrate using conventional micro-machining techniques; growing an oxide film layer on one or more portions of the MEMS device determined to apply a physical stress effective for control of a performance parameter of the MEMS device; performing initial measurements of the performance parameter; irradiating the oxide film layer to remove a predetermined portion thereof; and optionally performing post-irradiation measurements of the performance parameter.
The irradiating of the oxide film layer is optionally repeated to remove an additional predetermined portion thereof until the performance parameter of interest is trimmed within predetermined limits.